Disk disease is endemic. When a spinal disk in a patient is no longer serviceable, a fusion is often done. A fusion involves the removal of the degenerative vertebral disk, which had allowed for movement and rotation of the adjacent vertebrae relative to one another. The two vertebrae surrounding the disk are then joined and fixed, removing the mobility from that area. However, fusions have many disadvantages. They are destructive and have a significant failure rate. They encourage degenerative disease of the disks above and below and most often require donor bone, which causes its own set of complications.
The evolution of surgical treatment of other diseased joints has progressed from fusion, to debridement and resurfacing and then to joint replacement. The standard of care for surgical treatment of advanced disease of hip and knee joints is now joint replacement. However, the search for a working spinal disk replacement has not yielded as great of a success as of yet. The currently available experimental prostheses are not widely accepted.
There are a number of ball and socket type arrangements that have been developed for disk prostheses, but the problem with existing prostheses is that none of the devices address the need for self centering of the ball within the socket. Self-centering is an important feature because it allows the prosthesis to imitate the normal motion of the disk. The availability of a mechanism which enables a ball and socket type prosthesis to self-center would give a patient a flexible, natural-feeling prosthesis. An additional problem which is not addressed by the existing devices is any sort of specific method for building lordosis into the prosthesis. The advantage of building lordosis into the prosthesis is that it allows the back to have a more natural curvature, rather than an artificial stiffness. For a disk prosthesis to be as successful as some other joint replacements, it needs to allow for as much natural movement as possible.
It is also important that a prosthesis allows the anatomy of the motion segment to be the constraining factor for the limits of motion. If the prosthesis is intrinsically constrained, then it must bear the stresses of constraint, particularly at the bone/prosthesis interface, as well as internally in the prosthesis. Other fully constrained protheses, such as total knees or elbows, have failed. If a prosthesis was developed that allowed the anatomy of the motion segment to be the constraining factor, then the facets and soft tissue would bear the stresses of constraint allowing for longer life of the prosthesis. The life of the prosthesis should be long enough to make the operation worthwhile, so it need not be repetitive.
It is desirable that a prosthesis be provided that allows for a significant range of motion, that mimics the motion of an actual vertebral disk. Ideally, it would be stable, without intrinsic constraining factors, but not stiff, so it will have a long lifetime, and feel as natural as possible.